Components at the production line are very seldom found in pure state, as they rather form emulsions or heterogeneous blends which relative proportions change with time. Said condition is typically termed “multiphase”. This particularity of the fluid to be measured prevents use of conventional flow-meters, which may only measure total fluid flow-rate, as they are unable of identifying proportions of each particular component. Measurement errors are generally important because, among other factors, passage of gas in non-homogeneous proportions instantaneously modifies rheological properties of the multiple phase. The most used method actually consists of deviating production towards a temporal storage tank, collecting said generally daily production and, once components have separated by the action of gravity, relative volumes are measured. This method poses multiple implementing problems, as:
a) due to several reasons, it is not possible to completely separate blend components, such reasons may include:
1. formation of a water-oil emulsion interface, which may amount to a significant volume in the storage tank; and
2. natural settlement always results in a certain quantity of water remaining in emulsion with oil
b) even where both components could be adequately separated, devices used for interface measuring are difficult to implement, as, for example, it is not possible to establish water or oil levels by simple level inspection.
c) in fact, this technique only allows control of the individual production of each well on a monthly frequency at most.
As regards the real time application of measurement techniques in the oil production, special consideration should be given to the fact that gas tends to flow at higher speeds than liquid components. Thus, gas flow-rate measurements should be necessarily performed separately from liquid components, or else such flow velocity should be measured once all of the components of the blend are adequately mixed.
Other more sophisticated techniques have been invented which are based on different measuring principles. As an example we can mention the simple Venturi tube, the Coriolis principle, ultrasound, gamma rays measuring, and Magnetic Resonance (NMR).
The first technique is based on the measurement of the pressure difference existing between both ends of a tube with variable section. Measurements performed by means of this method strongly depend from the gas which is dispersed or bubbles within the blend. Further, this method does not discriminate the multiphase composition.
Coriolis's mass flow-meter is a mechanical design in which flow passage through a curved duct or other medium produces the vibration of mechanical parts thereof. Two or more vibration sensors are installed on the device, said sensors being positioned at a certain distance one from each other in the flow direction. Such flow produces vibrations at pre-established resonance frequencies which depend on the material and shape of said mechanical parts, but vary according to mass flow density (for more details see U.S. Pat. No. 4,187,721). It is also possible to derivate a phase difference between the resonance frequencies of both sensors, which angle, divided by the resonance frequency f, is proportional to the flow mass proportion (for more details see U.S. Pat. No. 5,648,616 or EP-A-866 319). As this method involves mechanical interactions, it is also strongly dependent on the fluid compressibility, which in turn strongly depends on the gas proportion, both that gas which is dissolved within the vein and that which bubbles through it.
Another design which does not exhibit moving mechanical parts is based on the ultrasound emission and reception in order to measure transit time of the fluid through the carrying vein. The time taken by the ultrasound wave in arriving to the receptor element since its emission and through the liquid flow, is proportional to the fluid speed within the carrying conduct. The fluid blend is obviously used as the coupling substance between the emitting crystal and the receptor. Here again, gas plays an essential role as regards the evaluation of measurement errors. On the one hand, bubbles break said coupling and introduce very important errors both as regards the propagation time of the acoustic signal and the attenuation of the sound wave. Even where bubbles are absent, compressibility of the liquid medium strongly depends from the quantity of gas dissolved therein, this being a variable which remarkably affects measurement process and result.
However, the Nuclear Magnetic Resonance principle allows both measurements to be done: i) determination of oil, gas and water proportions in the fluid blend, and ii) determination of the flow speed of said blend. There exist several patents which disclose methods—not necessarily selective as regards multiphase fluids—which use NMR analysis. Among them, the following can be mentioned: 1) Rollwitz et al., Method and Apparatus for Coal Analysis and Flow Measurements, U.S. Pat. No. 4,531,093; 2) King et al. Method and Apparatus for Measuring Flow in a Pipe of Conduit, U.S. Pat. No. 4,536,711, and 3) Reichwein, Consistency Measuring Device, U.S. Pat. No. 4,866,385.
Previous art devices designed for flow measuring and/or flow mapping are based on two widely known principles: 1) “time of flight” of saturated or unsaturated spins on the NMR spectrometer magnetic field; 2) on what is known as spatial codification of spins phase as they displace on a magnetic field gradient.
State of the art is completed with those patents which disclose specific methods for measuring flow-rate of multiphase fluids:
1. U.S. Pat. No. 4,785,245, entitled Rapid Pulse NMR Cut Meter describes a flow-meter which employs an NMR analysis in order to determine the fraction of one of the components of a multi-phase fluid flowing through a production line. NMR signal amplitude of a certain component is obtained by means of a pulse sequence which radio-frequency is adequate for the relative relaxation times between fluid components. This patent does not disclose a simultaneous method able to measure flow-rate of one of the phases which signal has separated. That is to say, it requires another device in order to measure flow velocity of said component.
2. U.S. Pat. No. 6,046,587, Measurements of Flow Fractions, Flow Velocities and Flow Rates of a Multiphase Fluid using NMR Sensing and U.S. Pat. No. 6,268,727, Measurements of Flow Fractions, Flow Velocities and Flow Rates of a Multiphase Fluid using ESR Sensing, both to J. D. King, Q. Ni and A. de los Santos, disclose a sensor which employs at least two NMR spectrometers or one NMR spectrometer and another ESR (Electronic Paramagnetic Resonance) one. Basic principle of measurement methodology is based on what is known as “time of flight” between both spectrometers. Such system of two spectrometers which separately measure residence time of each phase at the magnetic field is of unpractical and costly implementation; also, its application is difficult in the case of oil fields, which are generally subject to harsh climates. Another variant of said patents are those filed with the INPI (Argentine Industrial Property Institute) No. 010104816 (Oct. 12, 2001) and patent pending in USA 2004/0015332, by M. Ramia, D. J. Pusiol, C. A. Martin, E. Fried and R. Garnero. This case comprises a single electronic part which is shared by two sensing coils, operation principle being that already described, that is, flow velocity is measured through time of flight of water and oil molecules through the space between both sensing coils. This device exhibits the same restrictions as that involving two spectrometers: measurable flow-rate is substantially lower than 100 m3/day for the total fluid.
3. Flow-meter with phase separation disclosed by U.S. Pat. No. 6,452,390, by E. Wollin, entitled Magnetic Resonance Analyzing Flow Meter and Flow Measuring Method, proposes a methodology and associated apparatus which implementation is simpler than those already described. This methodology uses pulsed magnetic fields gradients in order to modulate the spins (or protons) precession phase. That is to say that spatial codification is carried out by which is commonly known as Laboratory System. The problem with this method is that at the common displacement velocities of protons on the magnetic field, magnetic field gradients application is technologically difficult to apply, due to the fact that, in order to produce an adequate magnetic field gradient, it is necessary to include important currents which on/off times are generally relatively long. That is to say that this methodology is generally restricted to relatively small flow-rates measurements.
4. Another variant of said patents are those filed with the INPI under No. P040102415 on Jul. 8, 2004, and the pending US patent publication no. 2006/0020403, by SpinLock SRL and D. J. Pusiol. Said applications disclose a flow-meter and cut sensor comprising a single coil associated to slanted planar plates magnet which generates a constant magnetic field, along with a magnetic field gradient. Said application further discloses the application of a pre-polarization field for rapid fluids, which may be removed in the case of slow fluids. Said application also discloses a derivation device with electronic key which allows measurement of N sensors groups with a single electronic system. In said application, spatial codification of the resonant nuclei position is carried out through a linear gradient of the magnetic field at the place of the excitation/Magnetic Resonance detection coil, which is preferably generated by the slanted position of the polar faces of a permanent magnet. In the case of high flow velocities, said gradient must be increased in order to attain the necessary effectiveness of the spatial codification process of the NMR spectrum of those protons forming the moving complex fluid. Upon the increase of said magnetic field gradient the Magnetic Resonance signal broadens and thus deteriorates. The maximum flow-rate limit measurable by means of this invention is given by the maximum value of magnetic field gradient that may be applied to the fluid passing through the sensor tube before the signal deteriorates, preventing the precision required for the particular use.
The inventive flow-meter employs a measurement method which is based on the passing time of the fluid molecules at a single sensor, without using any magnetic field gradient for fluid velocity measurements. This method consists of the ultrafast irradiation of hydrogen nuclei belonging to said molecules through repeated pulses within short time intervals, following a CPMG type sequence (see, e.g. A. Abragham, The Principles of Nuclear Magnetism, Oxford University Press, 1998); thus, the temporal evolution of the NMR signal depends from the number of “refreshment” molecules appearing in the volume of the excitation/detection coil. In turn, for a given interval between excitation pulses, said number of refreshment molecules depends on the fluid velocity at the production line. This invention is based on the fact that there exists a region of the CPMG sequence in which the spin echoes amplitude following application of each radio-frequency pulse exhibits a linear variation with time. Quotient between the slope of said linear relation and origin ordinate is proportional to flow velocity, whereas the origin ordinate contains information which allows the determination of the proportion of the different elements comprising the complex fluid, which produce a detectable NMR signal.
In order to establish the relative proportions within the heterogeneous blend of the production fluid (we are preferably referring to oil, water and gas proportions), the production duct is first introduced through a spins pre-polarization magnetic module and then through a magnetic resonance sensor module. Length of said pre-polarization field and the velocity of the complex fluid establish a passage time of said flow which, along with the relaxation time T1 of the spins species which form part of one of the components of the complex fluid, allows protons present in the complex fluid portion to attain a polarization sufficient to produce a magnetic resonance signal in the magnetic resonance sensor module. Polarization of a particular component on the pre-polarization field is selected by adjusting length thereof in such a way that its protons will attain enough polarization so as to provide a NMR signal. In order to select the Magnetic Resonance signal of the component exhibiting a longer relaxation time, there is added to the pre-polarization magnetic field a second field (temporarily pulsed), extending its action on the production vein. That is to say that, in this situation, ordinate at the origin of adjustment of the linear region of the echoes of the CPMG sequence, after a repeated pulses sequence, will now represent protons from two of the fluid components, each with higher and lower T1. From the quantitative comparison of ordinates at the origin of both signals, one which is obtained with a shorter length of the pre-polarization field and the other obtained with longer spatial length for the application of the pre-polarization field, relative proportions, or cuts, of both components are obtained. Where the fluid bears a third component, an adequately arranged third magnetic field will be added.
Measurements are sequentially carried out and appropriately repeated in order to obtain an adequate signal-to-noise relation. Typically, measurement is completed in a few seconds.
Accordingly, this invention may be applied in all those cases involving complex fluids circulation, as for instance: Oil-Water, Mud from Mining Operations, Industrial Fluids, etc.